1. Field of the Invention
The present invention relates to a switching power supply unit capable of reducing power consumption when a plurality of switching power supply units are used.
2. Description of the Related Art
Currently, switching power supply units are used for supplying power in various electrical devices. A switching power supply unit, for example, a switching power supply unit of a ringing choke converter type (RCC) or a quasi resonance type often adopts a method for reducing switching loss by causing an intermittent switching operation at times of light load. In this state, power consumption of a switching power supply unit is lowered. A switching power supply unit is generally provided with a normal mode for carrying out a continuous switching operation at the time of normal operation of an electrical device, and a standby mode for carrying out intermittent switching operation to suppress power consumption when an electrical device is standing-by.
Depending on the electrical device, there are also many cases where a two-power supply system is adopted, where a main power supply having a large output capacity and a sub power supply having a small output capacity are fitted, and switching is performed between output from the main power supply and output from the sub power supply in the normal mode and the standby mode, respectively.
With a control circuit for a conventional two-power supply system, at the time of normal operation (non-standby time) of the electrical device, as shown in FIG. 4, both the main power supply and the sub power supply are put into normal mode, and continuous switching operations are carried out. On the other hand, when the electrical device is in standby, the main power supply is switched into standby mode and intermittent switching is carried out, and by keeping only the sub power supply in normal mode supply of power is ensured. As a result, even in standby mode, the main power supply is in a state where intermittent switching is performed, which is the main hindrance to reduction in power consumption when standing by.